Flat panel electron beam displays comprise a cathode and an anode contained in an evacuated envelope. In operation, the cathode is held at a negative potential relative to the anode. Electrons are emitted from the cathode. The potential difference between the cathode and the anode accelerates the emitted electrons from the cathode towards the anode in a beam. A beam current thus flows between the anode and the cathode. In flat panel electron beam displays a matrix arrangement is disposed between the cathode and the anode. The matrix arrangement is formed by a pair of "combs" placed at right angles to each other. These are commonly referred to as rows and columns. Each pixel or subpixel lies at the intersection of a row and a column. Each of the combs has many separate elements which is comprised of either rows or columns. In operation, a control voltage is applied to each element of each of the combs. The control voltage applied to each element imposes an electrostatic force on the electron beam associated with that element whether it be a row or a column. The electron beam current associated with that element can be adjusted by adjusting the control voltage.
For displays that allow multiple levels of intensity to be displayed on the screen, the rows and columns perform distinct functions. The comb having rows (or horizontal lines) is used to set the bias conditions for the pixel, that is, it has a simple control voltage applied that switches the individual row conductors between an OFF (unbiased) state and an ON (biased) state. The circuit which provides this switching is simple and inexpensive. The comb having columns (or vertical lines) is used to control the brightness at which pixels which are biased ON will be displayed. The brightness is set at an analog level between being equivalent to the pixel being biased OFF and the maximum level which the display will support. The number of intermediate levels which are supported is determined by the circuits driving the columns. The analog drive circuits which provide this switching are relatively complex and expensive. In some displays the operation of the rows and columns may be transposed.
The analog drive circuits which drive the columns are usually implemented by the use of a Digital to Analog Converter (DAC) for each of the column conductors.
The display operates so that if an analog voltage from one of the column drivers (DACs) intersects with an OFF (or unbiased) row, then no beam current will flow, regardless of the analog output value of the DAC. If an analog voltage from one of the column drivers (DACs) intersects with an ON (or biased) row, then the pixel becomes active, and the beam current which flows will be determined by the DAC setting. It is the analog voltage from the DAC which determines how much beam current flows and hence, what intensity is displayed for that pixel. In a practical implementation, all of the DACs are driven in parallel with data for respective pixels in a given row, so that an entire row of data is presented simultaneously. The row which is active and displaying data propagates down the screen as the entire frame of data is constructed.
For a display having 1024 pixels in each row of the display, this means that 1024 separate DAC circuits are needed. The DAC circuits are relatively complex and expensive and the cost of these circuits is a significant proportion of the overall cost of the flat panel display. If a separate DAC is used for each of the three subpixels, associated with each of the three colours, that make up a pixel in a colour display, then 3072 DAC circuits are needed, which adds further to the overall cost of the flat panel display.